Add generic run plan executor contracts

[chubes4]

Summary

• Add a generic runtime-core RunPlanExecutor and worker adapter contract for bounded deterministic worker loops.
• Route agent fanout through an agent-task-named adapter so the generic executor stays domain-neutral.
• Add focused executor and fanout smoke coverage without adding WPSG/Data Machine concepts.

Tests

• npm run build
• npm run test:run-plan-executor
• npm run test:agent-fanout-executor
• npm run test:agent-task-contracts
• npm run test:run-registry

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Drafted the generic run-plan executor, agent-fanout adapter refactor, and focused tests; Chris to review and validate before merge.